The present invention relates to an umbilical for use in the offshore production of hydrocarbons, and in particular to a power umbilical for use in deep water applications.
An umbilical consists of a group of one or more types of elongated active umbilical elements, such as electrical cables, optical fiber cables, steel pipes and/or hoses, cabled together for flexibility, over-sheathed and, when applicable, armored for mechanical strength. Umbilicals are typically used for transmitting power, signals and fluids (for example for fluid injection, hydraulic power, gas release, etc.) to and from a subsea installation.
The umbilical cross-section is generally circular. The elongated elements of the umbilical are wound together either in a helical or in a S/Z pattern. In order to fill the interstitial voids between the various umbilical elements and obtain the desired configuration for the umbilical, filler components may be included within the voids.
ISO 13628-5/API (American Petroleum Institute) 17E “Specification for Subsea Umbilicals”, provides standards for the design and manufacture of such umbilicals.
Subsea umbilicals are installed at increasing water depths, commonly deeper than 2000 m. Such umbilicals have to be able to withstand severe loading conditions during their installation and their service life.
The main load bearing components in use for withstanding the axial loads due to the weight and to the movements of the umbilical are steel pipes (see U.S. Pat. No. 6,472,614, WO93/17176 and GB2316990), steel rods (see U.S. Pat. No. 6,472,614), composite rods (see WO2005/124213), tensile armor layers (see FIG. 1 of U.S. Pat. No. 6,472,614), or steel wire ropes.
The other elements of and in the umbilical, i.e. the electrical and optical cables, the thermoplastic hoses, the polymeric external sheath and the polymeric filler components do not contribute significantly to the tensile strength of the umbilical.
Electrical cables used in subsea umbilicals fall into two distinct categories respectively known as power cables and signal cables. Power cables are used for transmitting high electrical power (typically a few MW) to powerful subsea equipments such as pumps. Power cables are rated at voltages comprised between 1 kV and 200 kV, and generally between 6 kV and 35 kV (medium voltage range).
Signal cables are generally used for transmitting signals and low power (<1 kW) to electrical devices on the seabed. Signal cables are generally rated at a voltage smaller than 3000V, and typically smaller than 1000V.
The copper conductors of electrical cables are not load bearing components, because of the low tensile strength of copper. These copper conductors effectively add weight only to the umbilical. Unless protected, these electrical conductors may therefore be damaged by excessive elongation or crushing especially under severe conditions such as in deep water.
An object of the present invention is to solve this problem and provide a light weight/high strength umbilical comprising electrical power cables which can be used in deep water applications.